Optical filter and analytical instrument

ABSTRACT

An optical filter includes: a lower substrate; a lower mirror provided to the lower substrate; a lower electrode provided to the lower substrate; an upper substrate disposed so as to be opposed to the lower electrode; an upper mirror provided to the upper substrate, and opposed to the lower mirror; and an upper electrode provided to the upper substrate, and opposed to the lower electrode, wherein the upper substrate has a groove surrounding the upper mirror in a plan view, the groove includes a first side surface section, a second side surface section, a bottom surface section, a first end section located between the first side surface section and the bottom surface section, and a second end section located between the second side surface section and the bottom surface section, in a cross-sectional view, and the first end section and the second end section each have a curved surface.

BACKGROUND

1. Technical Field

The present invention relates to an optical filter and an analyticalinstrument.

2. Related Art

In the past, as an optical filter for selecting a light beam having atarget wavelength from the incident light beams and then emitting thelight beam, there has been known an optical filter of an air-gap typeand of an electrostatic-drive type, in which a pair of substrates aredisposed so as to be opposed to each other, each of the surfaces of thesubstrates opposed to each other is provided with a mirror, electrodesare respectively disposed in the peripheries of the mirrors, a diaphragmsection is disposed in the periphery of one of the mirrors, and thediaphragm section is displaced due to electrostatic force generatedbetween the electrodes to thereby vary the gap (air gap) between themirrors, thus taking out the light beam having a desired wavelength(e.g., JP-A-2003-57438).

In such an optical filter, it is required to control a gap between thepair of mirrors in an extremely narrow range such as a sub-micron rangethrough a range of several microns in the manufacturing process, andmoreover, it is important to keep the gap between the mirrors withaccuracy, and at the same time, to control the gap to have a desired gapamount.

Incidentally, in the optical filter of the air-gap type and of theelectrostatic-drive type, the diaphragm is moved by electrostaticattractive force to thereby vary the gap between the mirrors. Therefore,it results that the applied voltage for generating the electrostaticattractive force necessary for the displacement of the gap varies inaccordance with the thickness of the diaphragm section. Therefore, it isdesirable for the diaphragm section to be as thin as possible in orderfor holding down the applied voltage. However, thinning the diaphragmsection causes degradation of strength, and in the optical filterrepeatedly varying the gap stress is applied to the diaphragm sectionevery time the gap is varied, and therefore, there arises a problem thatthe degradation of strength directly causes breakage of the diaphragmsection.

SUMMARY

An advantages of some aspects of the invention is to provide an opticalfilter and an optical module equipped with the optical filter eachcapable of preventing the degradation of strength of the diaphragmsection even in the case of thinning the diaphragm section on the groundof holding down the applied voltage, and as a result, enhancing thestrength of the diaphragm section while reducing the maximum appliedvoltage, varying the gap stably, and being driven preferably.

According to an aspect of the invention, there is provided an opticalfilter including a lower substrate, a lower mirror provided to the lowersubstrate, a lower electrode provided to the lower substrate, an uppersubstrate disposed so as to be opposed to the lower substrate, an uppermirror provided to the upper substrate, and opposed to the lower mirror,and an upper electrode provided to the upper substrate, and opposed tothe lower electrode, wherein the upper substrate has a groovesurrounding the upper mirror in a plan view, the groove has a first sidesurface section, a second side surface section, a bottom surfacesection, a first end section located between the first side surfacesection and the bottom surface section, and a second end section locatedbetween the second side surface section and the bottom surface section,in a cross-sectional view, and the first end section and the second endsection each have a curved surface.

The optical filter according to this aspect of the invention has acurved surface in each of the end sections of the groove. Thus, thestress concentration to the end sections of the groove caused whenvarying the gap can be eased to thereby enhance the strength of thediaphragm section. As a result, a stable gap variation becomes possible,and it becomes possible to preferably drive the gap.

According to another aspect of the invention, in the optical filterdescribed above, the bottom surface section is flat, and the upperelectrode is disposed on the upper substrate within a region locatedunder the bottom surface section in the plan view.

Thus, it becomes possible to prevent a crack or the like in the upperelectrode due to the distortion of the groove from occurring.

According to another aspect of the invention, in the optical filterdescribed above, the first end section is located nearer to the uppermirror, and the first end section fails to overlap the upper mirror inthe plan view.

Thus, it is possible to prevent the propagation of the light beamentering the upper mirror from being blocked by the side surfacesections of the diaphragm, thereby making preferable sensing possible.

According to another aspect of the invention, in the optical filterdescribed above, the lower substrate and the upper substrate each have alight transmissive property.

By arranging that the first and second substrates each have a lighttransmissive property as described above, the transmittance of the lightbeam in the substrate is improved, and the strength of the light beamtaken out is also raised. Therefore, the efficiency of taking out thelight beam is improved.

According to another aspect of the invention, in the optical filterdescribed above, the groove is formed by performing a wet-etchingprocess after performing a dry-etching process.

By using the wet-etching process in the manufacturing process, itbecomes easy to provide a curved surface shape to the end section,thereby making it possible to ease the stress concentration to the endsections, and thus the strength of the diaphragm section can beimproved. Further, by combining the dry-etching process and thewet-etching process with each other, it becomes possible to reduce thetime necessary for forming the groove, and at the same time to provide astructure in which the end sections of the groove each have a curvedsurface, thus the stress concentration to the end sections can be easedto thereby improve the strength of the diaphragm section. As a result, astable gap variation becomes possible, and preferable drive becomespossible.

According to another aspect of the invention, there is provided ananalytical instrument using any one of the optical filters describedabove.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a plan view of an optical filter according to an embodiment ofthe invention.

FIG. 2 is a cross-sectional view of the optical filter according to theembodiment.

FIG. 3 is a diagram showing a relationship between the wavelength andthe transmittance in the case of applying no voltage in the opticalfilter according to the embodiment.

FIG. 4 is a diagram showing a relationship between the wavelength andthe transmittance in the case of applying a voltage in the opticalfilter according to the embodiment.

FIGS. 5A through 5C are diagrams for explaining a method ofmanufacturing the optical filter according to the embodiment.

FIGS. 6A and 6B are diagrams for explaining the method of manufacturingthe optical filter according to the embodiment.

FIGS. 7A through 7C are diagrams for explaining the method ofmanufacturing the optical filter according to the embodiment.

FIGS. 8A through 8C are diagrams for explaining the method ofmanufacturing the optical filter according to the embodiment.

FIGS. 9A and 9B are diagrams for explaining the method of manufacturingthe optical filter according to the embodiment.

DESCRIPTION OF AN EXEMPLARY EMBODIMENT

Hereinafter, an optical filter according to an embodiment of theinvention will be explained. Here, as the optical filter, an opticalfilter of an air-gap type and of an electrostatic-drive type will beexplained.

In FIGS. 1 and 2, the reference numeral 1 denotes an optical filter ofthe air-gap type and of the electrostatic-drive type. The optical filter1 is composed of an upper substrate 2, a lower substrate 3 bonded (orjoined with an adhesive) to the upper substrate 2 in the state of beingopposed thereto, a mirror 4A (an upper mirror) having a circular shapedisposed at a central portion of an opposed surface 2 a of the uppersubstrate 2, the opposed surface 2 a being opposed to the lowersubstrate 3, a mirror 4B (a lower mirror) having a circular shapedisposed at a central portion of a bottom surface of a first recessedsection 5 formed at a central portion of a surface of the lowersubstrate 3, the surface being opposed to the upper substrate 2, so asto be opposed to the mirror 4A via a first gap G1, an electrode 6A (anupper electrode) having a ring shape disposed in the periphery of themirror 4A of the upper substrate 2, an electrode 6B (a lower electrode)having a ring shape disposed in a second recessed section 7 having aring shape formed in the periphery of the first recessed section 5 ofthe lower substrate 3 so as to be opposed to the electrode 6A via asecond gap G2, and a thin-wall and ring-shaped diaphragm section 8provided to an opposite surface of the upper substrate 2 to the opposedsurface 2 a thereof, and formed at the position roughly corresponding tothe electrode 6A using an etching (selectively removing) process.

The diaphragm section 8 is composed of a first side surface section 8 c,a second side surface section 8 e, a bottom surface section 8 a, a firstend section 8 b located between the first side surface section 8 c andthe bottom surface section 8 a, and a second end section 8 d locatedbetween the second side surface section 8 e and the bottom surfacesection 8 a. Further, the first side surface section 8 b located near tothe mirror 4A in the diaphragm section 8 is formed so as not to overlapthe mirror 4A in a plan view. By adopting such a structure as describedabove, it is possible to prevent the propagation of the light beamentering the mirror 4A from being blocked by the first side surfacesection 8 c of the diaphragm section 8.

Further, the electrodes 6A, 6B disposed so as to be opposed to eachother via the second gap G2 and the diaphragm 8 constitute anelectrostatic actuator.

As the material of the upper substrate 2 and the lower substrate 3,glass can be used. As the glass, specifically, soda glass, crystallizedglass, quartz glass, lead glass, potassium glass, borosilicate glass,sodium borosilicate glass, alkali-free glass, and so on are preferablyused.

By using a material having a light transmissive property for both of theupper substrate 2 and the lower substrate 3, electromagnetic waves in adesired wavelength band or visible light can be used as the incidentlight.

Further, by forming both of the upper substrate 2 and the lowersubstrate 3 using a semiconductor material such as silicon, nearinfrared light can be used as the incident light.

The mirrors 4A, 4B are disposed so as to be opposed to each other viathe first gap G1, and are each composed of a dielectric multilayer filmhaving high-refractive index layers and low-refractive index layersstacked alternately to each other. It should be noted that the mirrors4A, 4B are not limited to the dielectric multilayer films, but alloyfilms having silver as a principal constituent or multilayer filmsthereof, for example, can also be used.

Out of these mirrors 4A, 4B, one 4A of the mirrors is provided to theupper substrate 2, which is deformable, and is therefore called amovable mirror, and the other 4B of the mirrors is disposed to the lowersubstrate 3, which is undeformable, and is therefore called a fixedmirror in some cases.

In the case of using the optical filter 1 in the visible light region orthe infrared light region, as the material of forming thehigh-refractive index layers in the dielectric multilayer film, titaniumoxide (Ti₂O), tantalum oxide (Ta₂O₅), niobium oxide (Nb₂O₅), and so oncan be used. Further, in the case of using the optical filter 1 in theultraviolet light region, as the material of forming the high-refractiveindex layers, aluminum oxide (Al₂O₃), hafnium oxide (HfO₂), zirconiumoxide (ZrO₂), thorium oxide (ThO₂), and so on can be used.

On the other hand, as the material for forming the low-refractive indexlayer in the dielectric multilayer film, magnesium fluoride (MgF₂),silicon oxide (SiO₂), and so on can be used.

The number of layers and thickness of the high-refractive index layersand the low-refractive index layers are appropriately set based on thenecessary optical characteristics. In general, in the case of forming areflecting film (a mirror) with a dielectric multilayer film, the numberof layers necessary for obtaining the optical characteristics is equalto or larger than 12.

The electrodes 6A, 6B are disposed so as to be opposed to each other viathe second gap G2, and for constituting a part of the electrostaticactuator for generating electrostatic force between the electrode 6A, 6Bin accordance with the drive voltage input thereto to thereby move themirrors 4A, 4B relatively to each other in the state in which themirrors are opposed to each other.

Thus, the electrodes 6A, 6B are arranged to displace the diaphragmsection 8 in a vertical direction in FIG. 2 to vary the first gap G1between the mirrors 4A, 4B, thereby emitting the light beam with awavelength corresponding to the first gap G1.

Further, as shown in FIG. 2, the electrode 6A is disposed within aregion located under the bottom surface section 8 a, which is a flatplane. If the electrode 6A is formed to overlap the first end section 8b and the second end section 8 d each having a curved surface, whendriving the diaphragm section 8, the diaphragm section 8 is distorted tocause large stress in the electrode 6A located under the first endsection 8 b and the second end section 8 d. Thus, there is a possibilitythat a problem such as a crack arises in the electrode 6A. However, byforming the electrode 6A within the region under the bottom surfacesection 8 a which is a flat plane, the crack and so on of the electrode6A due to the distortion of the diaphragm section 8 can be prevented.

It should be noted that since in the present embodiment the opposedsurface 2 a of the upper substrate 2 and the second recessed section 7provided to the lower substrate 3 are arranged to be parallel to eachother, the electrodes 6A, 6B are also parallel to each other.

It is sufficient for the material for forming the electrodes 6A, 6B tobe conductive, and the material is not particularly limited. However,metal such as Cr, Al, Al alloy, Ni, Zn, Ti, or Au, resin having carbonor titanium dispersed, silicon such as polycrystalline silicon(polysilicon) or amorphous silicon, or a transparent conductive materialsuch as silicon nitride or ITO can be used as the material.

As shown in FIG. 1, wiring lines 11A, 11B are connected respectively tothe electrodes 6A, 6B, and the electrodes 6A, 6B are connected to apower supply (not shown) via the wiring lines 11A, 11B.

It should be noted that the wiring lines 11A, 11B are formed in a wiringgroove 12A provided to the upper substrate 2 or a wiring groove 12Bprovided to the lower substrate 3. Therefore, it is arranged that thewiring lines do not interfere the bonding between the upper substrate 2and the lower substrate 3.

The power supply is for applying a voltage to the electrodes 6A, 6B as adrive signal to thereby drive the electrodes 6A, 6B, thus generatingdesired electrostatic force between the electrodes 6A, 6B. It should benoted that a control device (not shown) is connected to the powersupply, and it is arranged that by controlling the power supply usingthe control device, the electrical potential difference between theelectrodes 6A, 6B can be adjusted.

The diaphragm section 8 has a smaller thickness compared to a portion ofthe upper substrate 2 where the diaphragm section 8 is not formed. Theportion of the upper surface 2 with a smaller thickness compared to theother portion of the upper substrate 2 as described above is arranged tobe deformable (displaceable) with elasticity (flexibility), and thus,the diaphragm section 8 is arranged to have a wavelength selectionfunction for varying the first gap G1 to change the distance between themirrors 4A, 4B to a distance corresponding to a light beam with adesired wavelength, thereby outputting the light beam with the desiredwavelength.

The shape and the thickness of the diaphragm section 8 are notparticularly limited providing the light beams having wavelengths withina desired wavelength range can be output, and are specifically set inaccordance with the wavelength range of the output light beam requiredfor the optical filter 1 taking the variation amount, the variationspeed, and so on of the distance between the mirrors 4A, 4B intoconsideration.

In the optical filter 1 according to the present embodiment, in the casein which the control device and the power supply are not driven, andtherefore, no voltage is applied between the electrodes 6A, 6B, themirror 4A and the mirror 4B are opposed to each other via the first gapG1. Therefore, when a light beam enters the optical filter 1, it resultsthat the light beam with the wavelength corresponding to the first gapG1, for example, the light beam with the wavelength of 720 nm is outputas shown in FIG. 3.

Here, when driving the control device and the power supply to apply avoltage between the electrodes 6A, 6B, the electrostatic forcecorresponding to the level of the voltage (electrical potentialdifference) is generated between the electrodes 6A, 6B. As describedabove, the control device controls the power supply to thereby apply adesired voltage between the electrodes 6A, 6B, thus making it possibleto generate desired electrostatic force between the electrodes 6A, 6B.When the desired electrostatic force is generated between the electrodes6A, 6B in such a manner as described above, the electrodes 6A, 6B areattracted to each other due to the electrostatic force to thereby deformthe upper substrate 2 toward the lower substrate 3, and thus the firstgap G1 between the mirrors 4A, 4B is narrowed compared to the case inwhich no voltage is applied.

On this occasion, the stress is caused in the first end section 8 b andthe second end section 8 d by the movement of the diaphragm section 8due to the electrostatic force. However, according to the presentembodiment, since the first end section 8 b and the second end section 8d each have a shape with a large curvature radius, it becomes difficultto cause the stress concentration, the breakage is hardly caused even byrepeated drive of the diaphragm section 8, and thus preferable drive isrepeated.

Therefore, when a light beam enters the optical filter 1, it resultsthat the light beam with the wavelength corresponding to the displacedfirst gap G1, for example, the light beam with the wavelength of 590 nmis output, and the transmission wavelength is shifted toward a shorterwavelength as shown in FIG. 4.

Then, a method of manufacturing the optical filter 1 according to thepresent embodiment will be explained with reference to the accompanyingdrawings. FIGS. 5A through 5C, 6A, 6B, 7A through 7C, 8A through 8C, 9A,and 9B are cross-sectional views showing a method of manufacturing theoptical filter 1 according to the present embodiment.

The manufacturing method includes a manufacturing process of the uppersubstrate and a manufacturing process of the lower substrate.Hereinafter, each of the processes will sequentially be described.

1. Manufacturing Process of Upper Substrate

A shown in FIG. 5A, a mask layer 51 is deposited on the entire surfaceof the upper substrate 2. As a material for composing the mask layer 51,for example, a metal film made of Cr/Au or the like can be used. Thethickness of the mask layer 51 is not particularly limited, but ispreferably set to about 0.01 through 1 μm, further preferably about 0.1through 0.3 μm. If the mask layer 51 is too thin, the upper substrate 2may not sufficiently be protected, and if the mask layer 51 is toothick, the mask layer 51 may become easy to be peeled off due to theinternal stress of the mask layer 51. In the present embodiment, a Cr/Aufilm is deposited as the mask layer 51 by a sputtering process to have athickness of the Cr layer of 0.01 μm and a thickness of the Au layer of0.3 μm.

Subsequently, as shown in FIG. 5B, an opening section 51 a for formingthe diaphragm section 8 is provided to the mask layer 51. The openingsection 51 a can be formed by, for example, a photolithography process.Specifically, a resist layer (not shown) having a pattern correspondingto the opening section 51 a is formed on the mask layer 51, and then themask layer 51 is removed partially using the resist layer as a mask, andthen the resist layer is removed to thereby form the opening section 51a. It should be noted that the partial removal of the mask layer 51 isperformed by a wet-etching process or the like.

Subsequently, as shown in FIG. 5C, the upper substrate 2 is etched by awet-etching process to thereby form the diaphragm section 8. As anetching fluid, hydrofluoric acid or buffered hydrofluoric acid (BHF),for example, can be used. Further, alternatively, the diaphragm section8 can be formed by performing a wet-etching process after performing adry-etching process. According to this configuration, it is possible toreduce the time necessary for processing the groove, and at the sametime to form the structure in which the first end section 8 b and thesecond end section 8 d of the groove each have a curved surface, whichmakes it possible to ease the stress concentration to the end sectionsto thereby enhance the strength of the diaphragm section 8.

Subsequently, as shown in FIG. 6A, the electrode 6A and the wiring line11A are formed. As a material for forming the electrode 6A and thewiring line 11A, a metal film made of, for example, Cr, Al, or atransparent conductive material such as ITO can be used. The thicknessof the electrode 6A and the wiring line 11A is preferably set to, forexample, 0.1 through 0.2 μm.

In order for forming the electrode 6A and the wiring line 11A, the metalfilm or the like is deposited by a vapor deposition process, asputtering process, an ion-plating process or the like, and then thefilm is patterned by a photolithography process and an etching process.

Subsequently, the mirror 4A is formed at a position 2 a′ on the opposedsurface 2 a surrounded by the diaphragm section 8. For example, titaniumoxide (Ti₂O) as a material for forming the high-refractive index layerand silicon oxide (SiO₂) as a material for forming the low-refractiveindex layer are stacked to each other, and then these layers arepatterned by a liftoff process to thereby obtain the mirror 4A.

2. Manufacturing Process of Lower Substrate

As shown in FIG. 7A, a mask layer 61 is deposited on the opposed surface3 a of the lower substrate 3 opposed to the upper substrate 2. As amaterial for forming the mask layer 61, a typical resist material isused.

Subsequently, as shown in FIG. 7B, an opening section 61 a for formingthe second recessed section 7 is provided to the mask layer 61. Theopening section 61 a can be formed by a photolithography process.

Subsequently, as shown in FIG. 7C, the lower substrate 3 is etched by awet-etching process to form the second recessed section 7. As an etchingfluid, hydrofluoric acid or buffered hydrofluoric acid (BHF), forexample, can be used. It should be noted that the method of forming thesecond recessed section 7 is not limited to the wet-etching process, butother etching processes such as a dry-etching process can also be used.

Subsequently, after removing the mask layer 61 by the etching process,the first recessed section 5 is then formed in the same manner as theformation of the second recessed section 7. Specifically, a mask layer62 is deposited on the lower substrate 3, as shown in FIG. 8A, tothereby form an opening section 62 a for forming the first recessedsection 5. Subsequently, as shown in FIG. 8B, the lower substrate 3 isetched by a wet-etching process to form the first recessed section 5.Then, as shown in FIG. 8C, by removing the mask layer 62 by the etchingprocess, the lower substrate 3 provided with the first and secondrecessed sections 5, 7 can be obtained.

Subsequently, as shown in FIG. 9A, the electrode 6B and the wiring line11B are formed. As a material for forming the electrode 6B and thewiring line 11B, a metal film made of, for example, Cr, Al, or atransparent conductive material such as ITO can be used. The thicknessof the electrode 6B and the wiring line 11B is preferably set to, forexample, 0.1 through 0.2 μm.

In order for forming the electrode 6B and the wiring line 11B, the metalfilm or the like is deposited by a vapor deposition process, asputtering process, an ion-plating process or the like, and then thefilm is patterned by a photolithography process and an etching process.

Then, the mirror 4B is formed at a position opposed to the mirror 4Adisposed on the upper substrate 2. For example, titanium oxide (Ti₂O) asa material for forming the high-refractive index layer and silicon oxide(SiO₂) as a material for forming the low-refractive index layer arestacked to each other, and then these layers are patterned by a liftoffprocess to thereby obtain the mirror 4B.

As explained hereinabove, according to the optical filter of the presentembodiment, when varying the first gap G1 in order for selectivelytaking out the wavelength, the stress is caused in the first end section8 b and the second end section 8 d by the movement of the diaphragmsection 8. However, according to the present embodiment, since the firstend section 8 b and the second end section 8 d each have a shape with alarge curvature radius, it becomes difficult to cause the stressconcentration, the breakage is hardly caused even by repeated drive ofthe diaphragm section 8, and thus preferable drive is repeated.

It should be noted that the optical filter according to the inventioncan be applied to an analytical instrument such as a colorimeter formeasuring colors or a gas detector for measuring gasses.

The entire disclosure of Japanese Patent Application No. 2010-063923,filed Mar. 19, 2010 is expressly incorporated by reference herein.

1. An optical filter comprising: a lower substrate; a lower mirrorprovided to the lower substrate; a lower electrode provided to the lowersubstrate; an upper substrate disposed so as to be opposed to the lowersubstrate; an upper mirror provided to the upper substrate, and opposedto the lower mirror; and an upper electrode provided to the uppersubstrate, and opposed to the lower electrode, wherein the uppersubstrate has a groove surrounding the upper mirror in a plan view, thegroove includes a first side surface section, a second side surfacesection, a bottom surface section, a first end section located betweenthe first side surface section and the bottom surface section, and asecond end section located between the second side surface section andthe bottom surface section, in a cross-sectional view, and the first endsection and the second end section each have a curved surface.
 2. Theoptical filter according to claim 1, wherein the bottom surface sectionis flat, and the upper electrode is disposed on the upper substratewithin a region located under the bottom surface section in the planview.
 3. The optical filter according to claim 1, wherein the first endsection is located nearer to the upper mirror, and the first end sectionfails to overlap the upper mirror in the plan view.
 4. The opticalfilter according to claim 1, wherein the lower substrate and the uppersubstrate each have a light transmissive property.
 5. The optical filteraccording to claim 1, wherein the groove is formed by performing awet-etching process after performing a dry-etching process.
 6. Ananalytical instrument comprising the optical filter according toclaim
 1. 7. An analytical instrument comprising the optical filteraccording to claim
 2. 8. An analytical instrument comprising the opticalfilter according to claim
 3. 9. An analytical instrument comprising theoptical filter according to claim
 4. 10. An analytical instrumentcomprising the optical filter according to claim 5.